In recent years, deep oil fields (including gas fields) that had not previously received attention have been actively developed on a global scale because of high oil prices and the imminent exhaustion of oil resources predicted in the near future. The depth of such oil fields (or gas fields) is generally very large. Their high-temperature atmospheres containing CO2, Cl−, and the like are severe corrosive environments. Thus, oil country tubular goods used for drilling such oil fields and gas fields need to be composed of materials having high strength and corrosion resistance. Oil field development in cold climate areas is also increasing; hence, the materials are often required to have low-temperature toughness as well as high strength.
The development of such deep oil wells disadvantageously requires a high drilling cost. A technique for expanding a relatively small pipe in an oil well has recently been brought into practical use (for example, see Patent Documents 1 and 2). The employment of the technique results in a reduction in the cross-sectional area of a drilling hole, thus reducing drilling costs. However, the tubular goods are required to have excellent expandability.
Patent Document 1: PCT Japanese Translation Patent Publication No. 7-567010
Patent Document 2: WO98/00626